This invention relates generally to medical diagnostic apparatus and methodology, and more specifically relates to X-ray scanning apparatus of the type utilized in computerized tomography.
Within very recent years, a relatively enormous degree of interest has been evidenced on the part of medical diagnosticians in a field now widely known as computerized tomography. In a typical procedure utilized in computerized tomography (or CT), an X-ray source and detector means are positioned on opposite sides of the portion of the patient which is to be examined. In the prior art these paired elements are made to transit across the body portion to be examined, while the detectors measure the X-ray absorption at the plurality of transmission paths defined during the transit process. Periodically as well, the paired source and detector means are rotated to a differing angular orientation about the body, and the transit process repeated. A very high number of absorption values may be yielded by procedure of this type, and the relatively massive amounts of data thus accumulated may be processed by a digital computer--which cross-correlates the absorption values to thereby derive absorption values for a very high number of points (typically in the thousands) within the section of the body being scanned. This point by point data may then be combined to enable reconstruction of a matrix (visual or otherwise) which constitutes an accurate depiction of the density function of the bodily section examined. The skilled diagnostician, by considering one or more of such sections, may diagnose various bodily elements such as tumors, blood clots, cysts, hemorrhages and various abnormalities, which heretofore were detectable, if at all, only by much more cumbersome and, in many instances, more hazardous (from the viewpoint of the patient) techniques.
While apparatus of the aforementioned type have therefore represented powerful diagnostic tools, and have been deemed great advances in the radiography art, apparatus heretofore designed and commercially available have suffered from many of the shortcomings incident to first generation devices. Thus, for example, it may be noted that acquisition of the raw data obtained as an incident of the discussed techniques frequently entailed an undesirably long period--which among other things subjected a patient to both inconvenience and stress. The patient's inability to remain rigid for such a lengthy period, also could lead to blurring of the image sought to be obtained.
In a copending application of John M. Pavkovich and Craig S. Nunan, Ser. No. 643,894 filed on Dec. 23, 1975 entitled "Tomographic Apparatus and Method for Reconstructing Planar Slices from Non-absorbed Radiation", and as well in the similarly copending application of John M. Pavkovich entitled "Apparatus and Method for Reconstructing Data", filed on Dec. 23, 1975, under Ser. No. 643,896, both applications of which are assigned to the same assignee as is the present application, apparatus and methodology are disclosed which alleviate a number of the prior art problems, most notably including the lengthy period that has heretofore been involved in computer processing of the raw data provided by the detectors. The apparatus therein disclosed utilizes a fan beam source of radiation coupled with application of a convolution method of data reduction, with no intervening reordering of fan rays, to thereby eliminate the errors and delays in computation time which would otherwise be involved in such reordering. The radiation source and the detector means are positioned on opposite sides of the portion of the patient to be examined and these elements are made to rotate through a revolution or portion thereof while the detectors measure the radiation absorption at the plurality of transmission paths defined during the rotational process.
In tomographic scanning apparatus heretofore widely known in the art, the detectors most commonly utilized for responding to the X-ray source took the form of scintillation counters which in turn were coupled to photomultipliers for providing suitable signal output levels. Detectors of this type, however, are known to suffer from several significant deficiencies. The scintillation crystals, for example, display hysteresis effects, i.e., they retain a memory of their earlier excitation state. Further, the photomultipliers which are utilized as an adjunct of the scintillation crystals, are relatively unstable elements which require frequent maintenance and attention, and are, in addition, relatively expensive.
While ionization detectors are well known as measuring elements for detecting radiation in X-ray or similar systems, it has not heretofore been deemed practical or appropriate to incorporate devices of this type into scanning systems of the type considered herein. This is in view of what has been deemed a necessity for relatively long path lengths in the cell elements comprising such detectors. In general, a problem of that type can presumably be overcome by providing relatively high gas pressures in the detector cells; but heretofore acceptable designs have not been forthcoming.